The preparation of organic isothiocyanates and thiocyanates by the reaction of a reactive organic halide, sulfur and an alkali metal cyanide in an oxygenated organic solvent, such as an aliphatic alcohol or ketone, was reported by Searles, U.S. Pat. No. 2,462,433. The Searles process suffers from several disadvantages:
1. Removal of salt from the organic reaction mixture, PA1 2. Removal of solvent from the product, and PA1 3. Distillation of the desired organic isothiocyanate from the co-produced organic thiocyanate. PA1 (a) reacting in the presence of a phase transfer catalyst, an alkali metal or ammonium cyanide, sulfur, and an allyl halide in water as the reaction solvent to produce a reaction mixture comprising an organic phase containing an allyl isothiocyanate and an aqueous phase; PA1 (b) separating said aqueous phase from said organic phase; and PA1 (c) reacting said allyl isothiocyanate with an alcohol in the presence of a urethane catalyst to produce the N-allyl-O-alkyl thionocarbamate. PA1 (a) forming an aqueous solution of said alkali metal or ammonium thiocyanate by the addition of about 0.5 to about 1.0 stoichrometic percent excess of said sulfur to an aqueous solution of said alkali metal or ammonium cyanide where said phase transfer catalyst is present in the amount of about 0.05 to 4 mole percent based on said alkali metal or ammonium cyanide; PA1 (b) adding to said thiocyanate solution at a temperature of less than about 40.degree. C., about a 1.0 to 1.1 molar proportion based on said alkali metal or ammonium cyanide of allyl halide; and PA1 (c) heating the resulting reaction mixture at reflux for a period of time sufficient to achieve a ratio of allyl isothiocyanate to allylthiocyanate of at least 8.5:1. PA1 (a) adding water to the reaction mixture to dissolve salt formed in the reaction; PA1 (b) separating the aqueous phase from the organic phase; and PA1 (c) removing unreacted allyl halide and trace amounts of water from said organic phase by heating said phase in vacuo for a time, temperature, and vacuum sufficient to remove unreacted allyl halide and trace water. PA1 (a) adding to said organic phase containing said allyl isothiocyanate about 1 to about 2 molar proportions based on said allyl isothiocyanate of a linear or branched chain primary alcohol and from about 0.1 to 4 mole percent based on said allyl isothiocyanate of a urethane catalyst; and PA1 (b) heating the resulting mixture to a temperature sufficient and for a time sufficient to convert said allyl isothiocyanate to N-allyl-O-alkyl thionocarbamate.
When an alcoholic solvent is used in the Searles process and the salt is not removed following formation of the organic isothiocyanate, low yields of thioncarbamate are obtained in the subsequent reaction with an alcohol.
The further reaction of an organic isothiocyanate with an aliphatic alcohol under the influence of a suitable catalyst, such as ferric acetylacetonate or dibutyl tin dilaurate, is known and is described by Iwakura et al, Can. J. Chem. 40, 2369-2375 (1962).
Reeves et al, Synthetic Communications 6 (7), 509-514 (1976), disclose that the reaction of an alkyl halide with aqueous potassium thiocyanate under the influence of a phase transfer catalyst (quaternary ammonium halides) or n-butylamine or tri-n-butylamine, affords alkyl thiocyanates. When allyl halides were used, a mixture of allyl thiocyanate and allyl isothiocyanate was obtained, which requires distillation to recover the desired isothiocyanate.
Generally, the prior art processes provide many disadvantages, including the complicated removal of reaction solvent and unwanted reaction products, exposure to irritating intermediate and deficient yields. These and other deficiencies of the prior art processes are overcome by the process of the instant invention which provides for a process for the preparation of N-allyl-O-alkyl thionocarbamates which comprises:
The present invention provides many improvements over the prior art disclosed processes in that (1) water is used as a solvent for the preparation of the allyl isothiocyanate, (2) the alkali metal or ammonium thiocyanate solution is readily prepared in situ by the use of a phase transfer catalyst, which additionally provides high yields of the allyl isothiocyanate without distillation, (3) salt formed in the reaction is readily removed, (4) the allyl isothiocyanate need not be isolated from the reaction vessel in which it is prepared, thus eliminating the need for handling an irritating lacrymator, and (5) the preparation of the N-allyl-O-alkyl thionocarbamate is readily prepared in high yields by the addition of an aliphatic alcohol to the allyl isothiocyanate and reaction under the influence of a suitable catalyst. These and other advantages of the instant process will become more evident in the detailed disclosure that follows.